Most conventional methods for drilling into the earth employ either an abrasive drill, which usually includes rotating steel bits, or abrasive loaded high-pressure jets. Although the abrasive cutting method has been shown to be effective in many situations, this method must be used within 8-12" bore holes, and the total energy delivered to the rock face during the cutting operation is limited. While explosives can be used for moving large masses of material, the precise repetitive cutting action which is needed for drilling a hole cannot be achieved with explosives.
Recently, it has become possible to utilize a pulsed electric power system which can store energy of several kj to greater than 100 kj up to multiple megajoules. These energies correspond to the energy release of one to several sticks of dynamite and can be switched into a load in time scales of less than one microsecond. The energy levels and discharge times of these pulsed power systems are comparable to high velocity explosives. Additionally, these pulsed power systems can be recharged and repetitively fired at relatively high rates. This type of pulsed electric power system can be utilized in a focused shock drill which transmits power electrically to the drilling head rather than relying on many miles of twisting drill stem pipe.
The energy can be discharged in powerful underwater sparks at the cutting face of the drill. Furthermore, this drill employs no moving parts in the well shaft, other than a descending drill bit and circulating drill mud. This system can reduce costs and the time-consuming process of changing drill bits since the bit employed by this focused shock drilling method has a much longer life than conventional abrasive drill bits. Additionally, because the focused shock drilling system does not rely on the hardness of steel for fracturing rock, it may be more effective in penetrating hot rock than conventional drilling methods.
Typical prior art spark discharge shock drills are described in U.S. Pat. Nos. 2,953,353 to Allen; 3,158,207 to Rowley; 3,500,942 to Smith, Jr.; 3,506,076 to Angona; 3,679,007 to O'Hare; 3,708,022 to Woodruff and 3,840,078 to Allgood et al. However, these prior art patents which use a spark discharge to assist a conventional abrasive drill or as the sole drill bit have met with limited practical success since the power generated at the drill bit could not effectively compete with conventional abrasive rotary drills. This ineffectiveness results from the fact that spark energy must be discharged before the spark channel in the drilling fluid has time to significantly expand and reduce the spark impedance and hence limit the peak pressure wave which is formed. This occurs in typically much less than one microsecond. Conventional high-energy capacitors require one to several microseconds or longer to discharge and, therefore, the majority of the stored energy cannot be effectively used. Additionally, the electrical impedance of an underwater spark changes very rapidly with time, making efficient energy transfer from the storage capacitor quite difficult. Furthermore, these prior art spark discharge drills could not be employed to actively steer the underground hole which was being developed.